The invention relates to a series-wound motor having a stator and an armature, having at least one field winding which is connected in series with an armature winding, and having at least one permanent magnet, whose magnetic field is superimposed on the main field produced by the at least one field winding.
In principle, it is known in the prior art (cf. DD 109 775, DD 145 590, DE-PS1 181 309, EP 0 582 416 B1) to provide an auxiliary series winding for direct-current motors with permanent-magnet excitation, in order to prevent the motor from being demagnetized by the armature reaction.
Furthermore, DE 37 29 680 A1 discloses a series-wound permanent-magnet motor which has interpoles with two-part interpole compensation windings for each interpole, with the aim of improving the efficiency of the interpoles in this way, and of preventing the permanent magnets from being demagnetized by the armature reaction.
Furthermore, EP 0 615 331 A1 discloses a series-wound motor in the form of a universal motor having a braking device, in which self-excited braking is intended to be initiated reliably by the use of a permanent magnet. In this case, during motor operation, an operating field is produced via the field windings and, together with an armature field which is formed, results in a resulting field as required for motor operation. When switching to braking operation, the motor is disconnected from the power supply system, and the field windings are short-circuited via the armature. In order to ensure reliable initiation of braking by self-excitation, a permanent magnet is additionally arranged in a slot on the pole shoe laminated core, to be precise such that a magnetic field acts in the same direction as the lines of force of the electrically excited main flux. The strength of the permanent magnet is designed such that its field is at least twice as great as the field resulting from the remanent induction in the iron of the pole shoe laminated core. This means that, even in the situation in which the remanent induction at the switching time from motor operation to generator operation acts against the field of the permanent magnet, there is sufficient excess permanent magnetic field in order to reliably initiate self-excitation. In order to prevent the permanent magnet from being demagnetized when the motor is started, either a starting resistor is provided or a diode, thus allowing the starting current initially to flow only in one direction, until the diode is bridged after a certain switch-on delay, such that the motor is then supplied with current directly from the AC power supply system.
Series-wound motors are used in particular in the form of universal motors for numerous applications, in particular in electrical tools. However, series-wound motors have the disadvantage that the (unregulated) no-load rotation speed is considerably higher than the (unregulated) rated rotation speed when operated on load. This means that the rotation-speed/torque characteristic is relatively steep and that the rotation speed is therefore relatively highly dependent on the torque. This means that the use of series-wound motors is problematic in those applications in which the (unregulated) no-load rotation speed must be limited, for example for safety reasons.